1. Field of the Invention
The present invention relates to the field of cooling the content of a vessel using a cryogenic liquid.
It relates, in particular, to the cooling of food products in apparatuses of the blender, mixer or kneader type, it then being possible for the content of the apparatus to be solid or pasty, as is the case with meat, or else liquid.
2. Description of Related Art
Taking the example of meat blenders, it is known that there is considerable amount of literature regarding the use of liquid CO.sub.2. In particular, liquid CO.sub.2 is injected into the bottom part of the blender so as to improve the heat exchange conditions between the cryogenic liquid and the meat. Reference may be made, for example, to documents U.S. Pat. No. 4,476,686 and EP-744,578.
This use of liquid CO.sub.2 in meat blenders has since formed the subject of very many industrial applications throughout the world which have made it possible, in hindsight, to clearly demonstrate the existence of a number of drawbacks associated therewith, among which mention may be made of one drawback of a technical nature and one drawback of an economic nature:
from a technical standpoint, difficulties have arisen in the case of certain products which, after cooling, undergo a subsequent cooking operation. For example, in a fryer the product undergoes a "popcorn effect" as commonly referred to by those skilled in the art, wherein the meat is seen to fragment or sputter during frying.
Without being completely elucidated, this phenomenon has been at least partially explained as resulting from gaseous CO.sub.2 dissolved in the mass of meat.
From an economic standpoint, it should be pointed out that the sites where food products are processed traditionally employ several fluids, among which nitrogen, in one or more forms, including liquid form, is most commonly found.
It will therefore be understood that it would be a great advantage to replace liquid CO.sub.2 (as we have seen, the source of the "popcorn" effect) and to use a single source of cryogenic liquid, for example liquid nitrogen, for the various operations carried out on the site. This therefore requires the possibility of using liquid nitrogen, as a replacement for the traditional liquid CO.sub.2, in such operations as cooling the content of blenders, mixers and other kneaders.
However, it should be pointed out that liquid nitrogen, because of its very special properties (in particular those related to its very low temperature) poses technical difficulties in cooling operations, both with regard to obtaining good energy efficiency and good transfer of the sensitive heat of the gas and to ensuring good dispersion of the cryogenic liquid throughout the product mass, or else with regard to avoiding the negative consequences of a sudden cooling of the environment and of the treated product (risk of liquefaction of the oxygen in the air, undesirable deep freezing of the product or else the sticking of the product to be cooled to the walls of the blender or to the components for injecting the cryogenic fluid).
Documents GB-1,430,385 and EP-A-166,655 have already mentioned the advantage, in operations to cool liquid foods or detergent powders, by injecting liquid nitrogen into the bottom part of the vessel containing the product to be cooled.
Document EP-A-711,511 has also recently mentioned the problems associated with the use of liquid nitrogen in the bottom of food blenders and has proposed, by way of solution, separating the gas phase from the cryogenic liquid so as to inject a substantially neat liquid into the bottom of the blender and, advantageously using injectors in the form of rectangular slots.
It will be noted that this document solves the problem of any sticking of the product on the injectors and other pipes inside the blender by flushing them using the gas phase previously separated from the cryogenic liquid.
The studies carried out to a successful conclusion by the Applicant on this subject have shown that it is possible to solve the technical problems mentioned above and, in particular, to obtain excellent results both with regard to the homogeneity of the temperature reached in the product mass and with regard to eliminating the "popcorn" effect, as long as the cryogenic liquid is injected into the vessel at a significant pressure, this being at least 3 bar and preferably lying within the range of 5 to 15 bar.
Returning to the case of liquid nitrogen, these studies have therefore made it possible to demonstrate that it is advantageous here not to use the conventional low-pressure liquid-nitrogen storage tanks (the pressure being typically about 1.5 to 2 bar, or up to 3 bar), but rather the medium-pressure liquid-nitrogen storage tanks which specifically allow liquid nitrogen to be delivered at a pressure of greater than 3 bar.
It may be immediately noted that such a use of liquid nitrogen (or other cryogenic liquid) at high pressure may, from the thermal standpoint, seem to be extremely paradoxical (or even repugnant) to those skilled in the cryogenic art in such food cooling (refrigeration) applications.
This is because it should be considered that the temperature of liquid nitrogen under its conventional conditions of use (the pressure being typically about 1.5 to 2 bar) is about -190.degree. C., while at about 10 bar (the mid-point of the pressure range envisaged here), its temperature stabilizes to about -180.degree. C., hence a significant thermal loss (refrigeration).
However, it has thus been made possible to demonstrate a saving in the cycle time for cooling meat masses by switching from liquid CO.sub.2 to pressurized liquid nitrogen (at a pressure of between 5 and 15 bar), allowing the cycle time to be reduced by half.